This invention relates to the field of semiconductor lasers, and particularly relates to vertical cavity surface emitting lasers. More particularly, the invention relates to self-pulsing vertical cavity surface emitting lasers (VCSELs).
Conventional semiconductor lasers have found widespread use in modern technology as the light source of choice for various devices, e.g., communications systems, compact disc players, and so on. The typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers. The low bandgap layer is termed the "active layer", and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types and when current is passed through the structure, electrons and holes combine in the active layer to generate light.
Several types of surface emitting lasers have been developed. One such laser of special promise is termed a "vertical cavity surface emitting laser" (VCSEL). (See, for example, "Surface-emitting microlasers for photonic switching and interchip connections," Optical Engineering, 29, pp. 210-214, March 1990, for a description of this laser. For other examples, note U.S. Pat. No. 5,115,442, by Yong H. Lee et al., issued May 19, 1992, and entitled "Top-emitting surface emitting laser structures," which is hereby incorporated by reference, and U.S. patent application Ser. No. 08/175,016, by Mary K. Hibbs-Brenner, allowed, issue fee sent Mar. 20, 1995, and entitled "Integrated laser power monitor," which is hereby incorporated by reference, U.S. Pat. No. 5,475,701. Also, see "Top-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 .mu.m," Electronics Letters, 26, pp. 710-711, May 24, 1990.) The laser described has an active region with bulk or one or more quantum well layers. The quantum well layers are interleaved with barrier layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is turned on and off by varying the current through the active region. However, a technique for digitally turning the laser on and off, varying the intensity of the emitted radiation from a vertical cavity surface emitting laser by voltage, with fixed injected current, is desirable. Such control is available with a three terminal voltage-controlled VCSEL described in U.S. Pat. No. 5,056,098, by Philip J. Anthony et al., and issued Oct. 8, 1991, which is hereby incorporated by reference.
For several reasons, it is desirable to use surface emitting devices. For example, surface emitting devices can be fabricated in arrays with relative ease while edge emitting devices can not be as easily fabricated. An array of lasers can be fabricated by growing the desired layers on a substrate and then patterning the layers to form the array. Individual lasers may be separately connected with appropriate contacts. Such arrays are potentially useful in such diverse applications as, for example, image processing inter-chip communications, i.e., optical interconnects, and so forth. Second, typical edge-emitter lasers are turned on and off by varying the current flow through the device. This requires a relatively large change in the current through the device which is undesirable; the surface emitting laser, described below, requires lower drive current, and thus the change of current to switch the VCSEL need not be large.
High-yield, high performance VCSELs have been demonstrated, and expedited in commercialization. There have been demonstrated breakthroughs in record performance and flexibility exploiting variation of this VCSEL platform.
Top-surface-emitting AlGaAs-based VCSELs are producible in a manner analogous to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) giving very high device yields.
The flexibility of this technology was exploited for lateral mode engineering including spatially filtered hybrid semiconductor/dielectric DBR VCSELs for single-mode emission with stable wavelengths and current. At the other extreme, a "quasi-incoherent" (multi-wavelength) VCSELs have been demonstrated with properties that alleviate modal noise in multi-mode fibers to overcome mode selective loss, especially in data communication applications, or analogously noisy speckle patterns in imaging applications.